The flaw lies in the diagram we started with:
It is way of working out structures - no more than that. The problems arise when you try to take it too literally. The usual way of teaching this is an easy way of working out what the electronic structure of any atom is - with a few odd cases to learn like chromium or copper. Where is the flaw in what is usually taught? The explanations around ionisation energies are based on the 4s electrons having the higher energy, and so being removed first. We say that the first ionisation energies don't change much across the transition series, because each additional 3d electron more or less screens the 4s electrons from the extra proton in the nucleus. When you talk about ionisation energies for these elements, you talk in terms of the 4s electrons as the outer electrons being shielded from the nucleus by the inner 3d levels. Those statements are directly opposed to each other. So the 4s orbital must have a higher energy than the 3d orbitals. The electrons lost first will come from the highest energy level, furthest from the influence of the nucleus. We know that the 4s electrons are lost first during ionisation. We say that the 4s orbitals have a lower energy than the 3d, and so the 4s orbitals are filled first. This last bit about the formation of the ions is clearly unsatisfactory. The 4s electrons are lost first followed by one of the 3d electrons. To write the electronic structure for Fe 3+: Fe When d-block elements form ions, the 4s electrons are lost first. In all other respects, the 4s electrons are always the electrons you need to think about first. The reversed order of the 3d and 4s orbitals only seems to apply to building the atom up in the first place. In all the chemistry of the transition elements, the 4s orbital behaves as the outermost, highest energy orbital. This is probably the most unsatisfactory thing about this approach to the electronic structures of the d-block elements Making positive ions from the d-block ions To make the table look less complicated, I am using to represent 1s 22s 22p 63s 23p 6. Most chemistry books and chemistry teachers try to explain the breaks in the pattern at chromium and copper - but not very convincingly. Each additional electron you add usually goes into a 3d orbital. The electronic structures of the d-block elements are shown in the table below. (Actually, that turns out not to be true! We will come back to that in detail later.) Kĭ-block elements are thought of as elements in which the last electron to be added to the atom is in a d orbital. But if you refer back to the energies of the orbitals, you will see that the next lowest energy orbital is the 4s - so that fills first.
#CHROMIUM ELECTRON CONFIGURATION FULL#
I just want to focus on the fourth period.Įverything is straightforward up to this point, but the 3-level orbitals aren't all full - the 3d levels haven't been used yet. Similar confusion occurs at higher levels, with so much overlap between the energy levels that the 4f orbitals don't fill until after the 6s, for example. They are shown at a slightly higher level than the 4s - and so it is the 4s orbital which will fill first, followed by all the 3d orbitals and then the 4p orbitals. The oddity is the position of the 3d orbitals. The diagram (not to scale) summarises the energies of the orbitals up to the 4p level. Where there is a choice between orbitals of equal energy, they fill the orbitals singly as far as possible.
You shouldn't find anything unfamiliar in it.Įlectrons fill low energy orbitals (closer to the nucleus) before they fill higher energy ones. It is taken from things you have probably already read elsewhere on Chemguide. This section is just a summary of the way this is currently taught. This page takes a closer look at this, and offers a more accurate explanation which avoids the problems. However, it does throw up problems when you come to explain various properties of the transition elements. The way that the order of filling of orbitals is normally taught gives you an easy way of working out the electronic structures of elements. I am grateful to Dr Eric Scerri from UCLA, who pointed these problems out to me and provided me with some useful academic papers I wouldn't otherwise have been able to get hold of. This page looks at some of the problems with the usual way of explaining the electronic structures of the d-block elements based on the order of filling of the d and s orbitals.
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